Udvikling af rustbeskytter til undervognsbehandling baseret på vegetabilske råvarer.

Development of Anti-Corrosion Products for Car Cavities Based on Vegetable Raw Materials

Summary and conclusions

In Denmark, between 250,000 and 300,000 cars are anti-corrosion treated every year. An estimate is that approximately 3 to 4 million litres of anti-corrosion products are used for this purpose, most of them being mineral oil based. These products are harmful to the environment as they contain large amounts of solvents as well as persistent or slowly degradable components.

This project was undertaken to investigate the possibility of developing an alternative anti-corrosion product for car cavities based on vegetable raw materials and containing no solvents. An alternative anti-corrosion product needs to have an improved environmental profile compared to the conventional products, but at the same time it also needs to resemble them regarding the technical properties. This means that the alternative must fulfil exactly the same technical specifications as conventional anti-corrosion products. Furthermore, it should be usable on the equipment already applied in the anti-corrosion treatment centres.

A literature study has been made to clarify which raw materials could be used in a vegetable alternative, and a screening has been made on the environmental and health effects of selected raw materials in order to exclude the most harmful ones.

Traditional mineral oil based anti-corrosion products contain a basis oil, wax, fatty substances and corrosion inhibitors. These inhibitors are often metal containing sulfonated naphthalene based compounds. The basis oil contains mineral oil, which typically consists of both fractions that evaporate after application, and fractions that do not evaporate. It often also contains one or more volatile organic solvents. A vegetable based anti-corrosion product should basically be composed of the same components as the conventional products but if possible the raw material should be of vegetable origin instead of petrochemical. The basis oil can therefore probably be a vegetable oil or a modified vegetable oil. As vegetable oils are non-volatile, and as no solvents should be added, the alternative anti-corrosion product will not dry by evaporation. Instead it has to dry by oxidative drying, which is a relatively slow drying process. Adding driers can, though, accelerate the process.

The ability to adhere to metal surfaces is a very important property of anti-corrosion products. If an anti-corrosion product does not adhere well to the treated metal surface, it will run off and only leave a very thin film on the surface. The amount of active matter (corrosion inhibitors) present on the surface will therefore be very limited, and the product will not provide the required protection against corrosion, even though it contains a very efficient corrosion inhibitor. Products intended for use in cavities need, furthermore, to have a sufficiently low viscosity to be applied by spraying. They must also have a good penetration into small cracks and crevices to provide optimum corrosion protection.

In conventional anti-corrosion products it is almost always possible to obtain the necessary correlation between application, adhesion and penetration characteristics by adjusting the amount of solvent present in the basis oil. The products are thin at application because of the solvents. They are therefore easy to apply and penetrate well into small cracks and crevices. The solvents evaporate relatively shortly after application and leave a film of corrosion protection. The product gets thicker as it dries and a good adhesion is thereby obtained.

Based on the testing performed within the project the main conclusion is that a vegetable based alternative containing wax, fatty substances and driers does not possess a sufficient ability to adhere to metal surfaces, no matter which vegetable oil it is based on. The product simply dries too slowly and runs off the metal surface before an adhering film is formed. A better adhesion can be obtained by increasing the amount of wax and/or fatty substance, but that will most likely result in a decreased penetration and spraying capacity, as the alternative is not diluted with volatile solvents. Moreover, the amounts of wax and fatty substances that can be dissolved in the vegetable oils will probably be limited.

A fully developed well-functioning alternative anti-corrosion product was not obtained within this project, as there seems always to be a conflict between the adhesion and penetration abilities of a vegetable based anti-corrosion product containing no solvent. No actual testing of the anti-corrosion effects of a vegetable based alternative has therefore been performed. A screening has, though, been made, testing different types of corrosion inhibitors in the vegetable basis formulation. Film forming corrosion inhibitors were included as well. The conclusion from the screening was that a sufficient anti-corrosion effect cannot be obtained with any of the tested inhibitors within the vegetable alternative. This can be due to several factors. One possibility is that the inhibitors do not provide the required anti-corrosion effect. Another possibility is that because of insufficient adhesion the film thickness of the remaining product on the surface is far too small. The answer is probably a combination of the two possibilities. Tests throughout the project have shown that a good adhesion is the first criterion that must be fulfilled to obtain a sufficient protection against corrosion by an anti-corrosion product.

This project shows that it is both time-consuming and very difficult to develop a more environmentally friendly alternative capable of fulfilling the same standards as requested for the solvent-based products. Substituting one solvent with another is one thing, but developing a product containing no solvents or water, which must fulfil the same technical requirements as a solvent based product is very difficult and in certain cases maybe impossible.

Beside the product development problems directly related to the basic physico-chemical characteristics of the alternative products, problems getting the samples of the required raw material were also experienced throughout the project. This has not only impeded the development process, but it also complicated the environmental screening. It was often uncertain, which raw materials would actually arrive, and, furthermore, the MSDS (Material Safety Data Sheets) and other environmental-related information were often not available until receipt of the raw material.

Even though the project did not result in a well-functioning alternative anti-corrosion product, an LCA-screening was undertaken in order to clarify whether it would be rational, from an environmental point of view, to continue the work to develop a vegetable based anti-corrosion product. Two model formulations have been made, whose environmental profiles were compared to that of a conventional mineral oil based anti-corrosion product.

Based on the model formulations the LCA screening shows that the vegetable alternative in most areas (except acidification and eutrophication) has a better environmental profile than the conventional product. This estimate was obtained based on the assumption that the vegetable alternatives resemble the conventional product regarding the anti-corrosion effect in so far as their protection is as good and long lasting, but none of the alternatives could fulfil these criteria at the end of the project. A screening of the alternative anti-corrosion products’ ability to protect against corrosion showed that metal surfaces treated with vegetable alternatives began to corrode within few days after immersion into a saltwater solution, whereas metal surfaces treated with a conventional anti-corrosion product were protected for several months before any signs of corrosion appeared.

In order to make the observations from the LCA screening valid and to make the vegetable alternative usable as an anti-corrosion product, it is essential to enhance the anti-corrosive effect of the product. This should primarily be done by enhancing the adhesion abilities of the product, securing at the same time that the product still fulfils the penetration requirements.

At the end of the project a more traditional inhibitor was therefore tested superficially in the alternative formulation. The inhibitor was a sulfonated naphthalene based compound. This alternative formulation did furthermore contain an increased amount of wax compared to the other alternative formulations tested in the project. The formulation containing the traditional inhibitor showed a good adhesion on metal surfaces and did apparently give a better protection against corrosion but the latter should be investigated more thoroughly. The ability of this formulation to penetrate cracks and crevices needs to be investigated as well, but it is expected that the formulation cannot fulfil the demands regarding penetration due to the increased amount of wax.

Even though a more promising formulation with regard to adhesion was obtained in the end of the project it does seem rather doubtful whether a vegetable anti-corrosion product can be developed with satisfying characteristics without using organic volatile solvents or adding water. The most likely way to solve the technical conflict between proper adhesion, good application abilities and good penetration and at the same time securing an improved environmental profile would be to try to develop water emulsion products based on the most promising of the tested vegetable formulations.